This invention relates to elevator buckets, and more particularly to a cantilevered bucket for a mill duty elevator.
A mill duty bucket elevator is an equipment system for lifting dense, flowable substances from a low elevation to a higher elevation. It is designed to withstand the severe service conditions imposed by the handling of materials such as cement, rock, fertilizer, lime, gypsum, pyrites, slag, coal, and fine ore.
A bucket elevator is made of a long endless chain loop trained around an upper traction wheel and a lower sprocket wheel. A series of buckets is attached to the chain at regular intervals for conveying material from one end of the elevator to the other end. The elevator is usually vertically oriented and operates in a casing having a feed chute for filling the buckets near the bottom or boot of the casing, and a discharge chute into which the buckets discharge their contents near the top or the head of the casing. The maximum height of a elevator is determined by the weight and strength of the chain, the density of the material to be carried, and the size and weight of the buckets.
Prior art elevator buckets have, in some circumstances experienced a short fatigue life. We have concluded, after analysis and testing, that the cause of fatigue failure in elevator buckets is due to resonant vibration of the buckets caused by impact of the chain bushings with the traction wheel.
The resonant vibration in the bucket, as it passes over the traction wheel, causes internal stresses in the bucket which are concentrated at several regions in the bucket. These stresses result in the initiation of fatigue cracks at these points of high stress, and these cracks propagate through the joints and corners of the bucket adjacent to its attachment point on the chain. Although no liability attaches to the manufacturer because of the use in conditions exceeding the specified limits for which the elevator was designed, these situations can create bad will for the company. Accordingly, it is considered desirable to design the buckets in such a way that they are resistant to fatigue cracking under all conditions of use.
One technique for lengthening the service life of elevator buckets is to provide vibration isolation mounting devices between the bucket and the chain. This scheme could probably be made to work, but suitable vibration isolation mounts for this application are excessively costly considering the number of buckets in an elevator, which can amount to as many as 400 buckets on a single chain. Therefore, we decided that a less costly and more durable solution to the fatigue cracking problem would be a redesign of the bucket itself to better enable it to withstand the vibration without developing fatigue cracks at points of stress.
One problem with bucket redesign for an existing elevator configuration is that the buckets must be compatible with the existing elevator design. This is because it would be extremely costly to redesign an elevator for a new bucket configuration. More importantly, the new bucket should be compatible with the thousands of existing elevators, so that when the buckets of the present design in these existing elevators wear out, they may be replaced with the improved buckets to improve the performances and operation of the old elevators.
In addition to improving the fatigue life, it would be desirable to improve the buckets in other ways. For example, the redesigned bucket should be easier and faster to fabricate, use lighter gauge materials, have fewer different pieces, and more easily meet critical dimensions. The improved bucket should be lighter, thereby making it possible to build elevators of greater height. Finally, if possible, the improved bucket should provide a larger capacity bucket while remaining dimensionally compatible with the existing elevators, and thereby improve the capacity of the elevator.